Chap. 12 Freezing Precipitation and Ice Storms

Learning objectives 1.Explain the processes by which freezing precipitation forms 2.Recognize on a sounding the atmospheric thermal structure that supports formation of the different types of freezing (or frozen) precipitation 3.Identify the synoptic patterns of fronts and pressure centers that are conducive to freezing precipitation events in the U.S. 4.Sketch on a map the North American distribution of freezing precipitation 5.Explain the hazards, both at the surface and aloft, associated with freezing precipitation

What is freezing precipitation* (FP)? Defined as: Rain or drizzle (supercooled – to be defined) that freezes on surfaces and causes an ice glaze Between , FP storms have produced $16B in damage, averaging $300M per year FP aloft – aircraft icing FP at the surface – ice storms –Structural damage or ice accumulation of 0.25 inch *Freezing precipitation is different from “frozen” precipitation such as sleet and snow. This is already frozen when it reaches the surface.

Supercooled water drops Existence of water (drops) in a liquid state at T < 0  C Dependence on size –Supercooled water is more likely for small drops Ice nuclei (IN) are required to freeze supercooled drops –IN are aerosol particles that activate (initiate) freezing –IN are typically not active for T > -7  C Question for discussion: Can freshwater lakes, ponds, and puddles be supercooled?

Vertical structure of the atmosphere during FP events Source of the drops in freezing rain: melted snow flakes –Melting requires an atmospheric layer > 0 °C (Fig. 12.1) –Freezing rain is associated with nimbostratus clouds Source of drizzle in freezing drizzle: warm rain process (ice phase is not involved), growth by condensation on cloud condensation nuclei –Source near the top of shallow (~1 km deep) clouds (Fig. 12.2) –Drizzle drops grow as they fall downward thru the cloud –Freezing drizzle is often associated with stratocumulus clouds

Production of freezing/frozen precipitation Precipitation forms as snow, which melts at lower levels and then refreezes when the supercooled drop strikes the surface. –Typical for freezing rain. –The surface temperature is < 0 °C For freezing drizzle, drops form in liquid form (warm rain process) and then freeze. –This is more likely to be small drops, such as drizzle. (Smaller drops remain supercooled for a longer time period.) Generation of freezing rain: see Fig. 12.1

Temperature profiles associated with freezing rain Four soundings at various locations across a warm front within a midlatitude cyclone. Fig. 12.1: This shows generation of snow, ice pellets, freezing rain, and rain. The temperature profile is important. What about mixtures of rain & snow?

Temperature profiles associated with freezing rain Four soundings at various locations across a warm front within a midlatitude cyclone. Fig. 12.1: This shows generation of snow, ice pellets, freezing rain, and rain. The temperature profile is important. What about mixtures of rain & snow?

Freezing drizzle Light precipitation (small drops in the form of drizzle, 0.2 mm in diameter) that freezes on surfaces and causes an ice glaze Total accumulations much lower than FR Aloft – aircraft icing – is a potentially big danger Forms in relatively shallow clouds with cloud tops warmer than -10 °C

Fig Situation for production of freezing drizzle. These clouds are relatively shallow and also present icing hazards to aircraft All supercooled water Smallest droplets here Largest droplets fall from cloud Drizzle ( mm diameter) (Important to prevent freezing) Hazard zone The drizzle does not freeze in the cloud because ice nuclei are not active or common for T > -7 C Drop growth

Fig In-cloud and subcloud icing present hazards to aircraft See Storms and Society, page 212. Roselawn aircraft accident due to freezing drizzle. 1)Added weight 2)Change in airflow around wings

Relatively narrow bands of freezing rain are produced for each case. Cyclone/Arctic high Need to get cold air southward via the push by the anticyclone Which of these is most common for freezing rain in the SE US? Fig Large-scale weather patterns associated with freezing precip. anticycloneCyclone alone Cyclone + arctic anticylone West flank of anticyclone

This one! The anticyclone pushes very cold air into the Southeast. A cyclone develops and advects warm moist air over the stationary front. Very high ice accumulations can result (>2 inches). Such storms produce extensive damage.

Damage from ice storms a)Weight of the ice* b)Wind (see “Wind and Ice Storms” box on p. 213) Increased stress and oscillating motions Ice + wind can be devastating *Quantitative question: What is the weight of ice on a single electric line over a span of 50 meters? Assume an ice accumulation of 1 cm, uniform around the cable.

Extreme Event 4-9 Jan 1998 $3B damage

Fun fact, p. 214 Warm Air Aloft! Very shallow arctic air mass exists below 900 mb. The inversion strength (difference in temperature between the base and top) is large – about 24 C in this case. The air mass above is conditionally unstable and would be supportive of thunderstorms and heavy rain (and hence large ice accumulations). Shallow cold air mass

Fig Weather patterns associated with freezing rain within the Appalachian Mountains Freezing rain associated with east coast events – 3 types

Fig. 12.5b Freezing rain associated with east coast events – 3 types

Fig. 12.5c Freezing rain associated with east coast events – 3 types

Fig Climatology of freezing rain. Hours per year at sites that receive freezing rain, freezing drizzle, and ice pellets. Fig Climatology of freezing rain. Hours per year at sites that receive freezing rain, freezing drizzle, and ice pellets. Infrequent over Southeast Rare occurrence over wester U.S.

Fig Median number of hours per year of freezing drizzle. Significant difference from location of freezing rain events. Why?

Determining Precipitation Type Rain (R, RA)- Rain is liquid precipitation that reaches the surface in the form of drops that are greater than 0.5 millimeters in diameter. The intensity of rain is determined by the accumulation over a given time. Categories of rain are light, moderate and heavy.

Cold Rain Sounding

Determining Precipitation Type Snow (SN, SNW, S)- Snow is an aggregate of ice crystals that form into flakes. Snow forms at temperatures below freezing. For snow to reach the earth's surface the entire temperature profile in the troposphere needs to be at or below freezing. It can be slightly above freezing in some layers if the layer is not warm or deep enough the melt the snow flakes much. The intensity of snow is determined by the accumulation over a given time. Categories of snow are light, moderate and heavy.

Snow Sounding

Determining Precipitation Type Sleet / Ice Pellets (PE, PL, IP, SLT)- Sleet (Ice Pellets) are frozen raindrops that strike the earth's surface. In a sleet situation the precipitation aloft when it is first generated will be snow. The snow falls through a layer that is a little above freezing and the snow partially melts. If the snow completely melts it will be more likely to reach the earth's surface as supercooled water instead of sleet. If the snow partially melts there will still be ice within the falling drop for water to freeze on when the drop falls into a subfreezing layer. The lowest layer of the troposphere will be below freezing in a sleet situation and deep enough to freeze drops completely.

Sleet Sounding

Determining Precipitation Type Graupel (GS)- Graupel forms in the same way as hail except the diameter is less than 5 millimeters. It usually grows by soft hail processes. Drizzle (DZ, L)- Drizzle is liquid precipitation that reaches the surface in the form of drops that are less than 0.5 millimeters in diameter.

Determining Precipitation Type Freezing Drizzle (FZDZ, ZL)- Freezing Drizzle is liquid precipitation that reaches the surface in the form of drops that are less than 0.5 millimeters in diameter. The drops then freeze on the earth's surface. Freezing Rain (FZRA, ZR)- Freezing Rain is liquid precipitation that reaches the surface in the form of drops that are greater than 0.5 millimeters in diameter. The drops then freeze on the earth's surface.

Freezing Drizzle Sounding

Freezing Drizzle / Freezing Rain Sounding

Freezing Rain Sounding

Determining Precipitation Type Freezing Fog (FZFG)- Freezing fog is a fog composed of supercooled water drops. These drops freeze just after they wet the earth's surface. Mixed Precipitation (MXD PCPN)- The combination of two or more winter precipitation types occurring at the same time or over a period of time at the same place.

Homework Test Your Understanding –1, 2, 5, 6, 9, 10 Test Your Problem Solving Skills –3